Density meters are commonly used instruments in industrial processes. Common types of density meters include nuclear densitometers, vibrating vane densitometers and Coriolis flow meters having a density measurement as a by-product measurement.
In most applications, density measurements are used to discern bulk properties of the process fluid. Typically, density measurements are intended to provide information about the liquid and solid phases of a process fluid. These measurements get confounded when an unknown amount of entrained air is present.
For a two-component mixture, knowing the component densities and accurately measuring the mixture density provides a means to determine the phase fractions of each of the two components. However, the presence of a third phase, such as entrained air (or gas) confounds this relationship. Typically, there is not significant contrast in the densities of the liquid components, which results in large errors in phase fraction determination resulting from small levels of entrained air.
The measurement of slurries used in the paper and pulp industries and in other industries particularly presents problems in the production of paper. Slurries commonly used in the paper and pulp industry are mostly water and typically contain between 1% and 10% pulp content by mass. Monitoring the gas volume fraction of a slurry can lead to improved quality and efficiency of the paper production process.
Processes run in the paper and pulp industry can often, either intentionally or unintentionally, entrain gas/air. Typically, this entrained air results in measurement errors in process monitoring equipment such as density meters. Industry estimates indicate that entrained air levels of 2-4% are common. Since most density meters are unable to distinguish between air and liquid, interpreting their output as a density measurement or composition measurement would result in an overestimate of the density of the liquid or slurry present at the measurement location. Similarly, the void fraction of the air within the pipe can cause errors in compositional measurements.
Thus, providing a method and apparatus for measuring entrained air in paper and pulp slurries, for example, would provide an accurate measurement of the entrained air and would provide a means to correct the output of density meters.
As suggested, multiphase process flow rate is a critical process control parameter for the paper and pulp industry. Knowing the amounts of liquid, solids and entrained gases flowing in process lines is key to optimizing the overall papermaking process. Unfortunately, significant challenges remain in achieving accurate, reliable, and economical monitoring of multiphase flow rates of paper and pulp slurries. Reliability challenges arise due to the corrosive and erosive properties of the slurry. Accuracy challenges stem from the multiphase nature of the slurries. Economical challenges arise from the need to reduce total lifetime cost of flow measurement, considering installation and maintenance costs in addition to the initial cost of the equipment.
Currently, there is an unmet need for multiphase flow measurement in the processing industry, such as the paper and pulp industry. Real time flow measurement is typically restricted to monitoring the total volumetric flow rate in a process line without providing information on the composition of the process mixture.
Similarly, well head monitoring represents a difficult technical challenge with the presence of entrained gas. Metering well head production rates is a long standing challenge for the oil and gas industry. Performing accurate and timely monitoring of the production rates has many benefits, which include optimizing overall field and specific well production. The difficulty is due in no small part to the extreme variability of produced fluids which can include various types and mixtures of oil, water, gas, and solid particles.
Many companies have developed various types of three phase meters designed to address the well head flow metering market. These products have met relatively limited commercial success due to a combination of performance, accuracy, and cost issues. This disclosure provide a means and apparatus for well head monitoring that combines multiple existing technologies in to system that should meet a wide range of cost and performance goals.
It is proposed herein to use sonar-based entrained gas measurement to determine the entrained gas level in conjunction with any mixture density measurement to improve the accuracy and therefore value of the density measurement. A sound speed based entrained gas measurement can accurately determine the entrained gas in an aerated mixture without precise knowledge of the composition of either the non-gas components of the multiphase mixture of the composition of gas itself. Thus, the entrained gas levels can be determined essentially independent of the determination of the liquid properties. The accuracy could be improved using the sound speed measurement and mixture density simultaneously, but is not required. Determining the entrained gas level enables the density measurement to determine the properties of the non-gas component of the multiphase mixture with the same precision as if the gas was not present. This capability also enables the density meters to provide significantly enhanced compositional information for aerated mixtures.